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row of three circular images appears at the bottom of the route.
Tapping any of the circular images flies that image to a new route
that displays it with a square shape.
Tapping the square image flies the hero back to
the original route, displayed with a circular shape.Before moving to the sections specific to
standard
or radial hero animations,
read basic structure of a hero animation
to learn how to structure hero animation code,
and behind the scenes to understand
how Flutter performs a hero animation.<topic_end>
<topic_start>
Basic structure of a hero animation
<topic_end>
<topic_start>What's the point?
Terminology:
If the concept of tweens or tweening is new to you,
see the Animations in Flutter tutorial.Hero animations are implemented using two Hero
widgets: one describing the widget in the source route,
and another describing the widget in the destination route.
From the user’s point of view, the hero appears to be shared, and
only the programmer needs to understand this implementation detail.
Hero animation code has the following structure:Flutter calculates the tween that animates the Hero’s bounds from
the starting point to the endpoint (interpolating size and position),
and performs the animation in an overlay.The next section describes Flutter’s process in greater detail.<topic_end>
<topic_start>
Behind the scenes
The following describes how Flutter performs the
transition from one route to another.Before transition, the source hero waits in the source
route’s widget tree. The destination route does not yet exist,
and the overlay is empty.Pushing a route to the Navigator triggers the animation.
At t=0.0, Flutter does the following:Calculates the destination hero’s path, offscreen,
using the curved motion as described in the Material
motion spec. Flutter now knows where the hero ends up.Places the destination hero in the overlay,
at the same location and size as the source hero.
Adding a hero to the overlay changes its Z-order so that it
appears on top of all routes.Moves the source hero offscreen.As the hero flies, its rectangular bounds are animated using
Tween<Rect>, specified in Hero’s
createRectTween property.
By default, Flutter uses an instance of
MaterialRectArcTween, which animates the
rectangle’s opposing corners along a curved path.
(See Radial hero animations for an example
that uses a different Tween animation.)When the flight completes:Flutter moves the hero widget from the overlay to
the destination route. The overlay is now empty.The destination hero appears in its final position
in the destination route.The source hero is restored to its route.Popping the route performs the same process,
animating the hero back to its size
and location in the source route.<topic_end>
<topic_start>
Essential classes
The examples in this guide use the following classes to
implement hero animations:<topic_end>
<topic_start>
Standard hero animations
<topic_end>
<topic_start>What's the point?
Standard hero animation codeEach of the following examples demonstrates flying an image from one
route to another. This guide describes the first example.<topic_end>
<topic_start>
What’s going on?
Flying an image from one route to another is easy to implement
using Flutter’s hero widget. When using MaterialPageRoute
to specify the new route, the image flies along a curved path,
as described by the Material Design motion spec.Create a new Flutter example and
update it using the files from the hero_animation.To run the example:<topic_end>
<topic_start>
PhotoHero class
The custom PhotoHero class maintains the hero,
and its size, image, and behavior when tapped.
The PhotoHero builds the following widget tree:Here’s the code:Key information:<topic_end>
<topic_start>
HeroAnimation class
The HeroAnimation class creates the source and destination
PhotoHeroes, and sets up the transition.Here’s the code:Key information:<topic_end>
<topic_start>
Radial hero animations
<topic_end>
<topic_start>What's the point?
Flying a hero from one route to another as it transforms
from a circular shape to a rectangular shape is a slick
effect that you can implement using Hero widgets.
To accomplish this, the code animates the intersection of
two clip shapes: a circle and a square.
Throughout the animation, the circle clip (and the image)
scales from minRadius to maxRadius, while the square
clip maintains constant size. At the same time,
the image flies from its position in the source route to its
position in the destination route. For visual examples
of this transition, see Radial transformation
in the Material motion spec.This animation might seem complex (and it is), but you can customize the
provided example to your needs. The heavy lifting is done for you.Radial hero animation codeEach of the following examples demonstrates a radial hero animation.
This guide describes the first example.Pro tip:
The radial hero animation involves intersecting a round shape with
a square shape. This can be hard to see, even when slowing
the animation with timeDilation, so you might consider enabling
the debugPaintSizeEnabled flag during development.<topic_end>
<topic_start>
What’s going on?
The following diagram shows the clipped image at the beginning
(t = 0.0), and the end (t = 1.0) of the animation.The blue gradient (representing the image), indicates where the clip